As is well known, the manufacture of semiconductor devices utilizing integrated circuits involves a diffusion step wherein various kinds of dopants form doped areas having p type or n type electroconductivity. In practicing this diffusion step, certain methods have been used, including gas-diffusion, solid-diffusion, ion implantation and coating diffusion. The coating diffusion method is carried out by coating the substrate's surface with a dopant source in a liquid form followed by calcination so as to form the desired doped areas. Despite the simplicity of the procedure, this method provides a doped area having a relatively high dopant concentration. The diffusion coating technique has thus been widely used in recent years.
In the manufacture of bipolar integrated circuit devices, the series resistance of the collector can be decreased by forming a buried collector layer within the semiconductor substrate. The epitaxial layer is thereafter formed on this semiconductor substrate. Arsenic and antimony are dopants which can be used to obtain diffusion into the semiconductor substrate to form a buried collector layer. Antimony is preferred to arsenic, however, antimony diffuses to a lesser degree from the buried layer to the epitaxial layer.
The most widely used coating liquid for the diffusion coating of antimony compounds is a silica-based, film-forming liquid coating containing antimony trichloride (SbCl.sub.3). The liquid coating is prepared by dissolving, in an organic solvent, a silanol compound represented by the general formula R.sub.r-n Si(OH).sub.n, in which R is a hydrogen atom, an alkyl group, an aryl group or an acyloxy group and n is a positive integer not exceeding 4; and a chlorinated antimony compound such as SbCl.sub.3, SbOCl, Sb.sub.4 O.sub.5 Cl and the like. Although these chlorinated antimony compounds are advantageous due to the relatively high concentration of the dopant which they supply when used as the dopant source, one of the serious problems in the use of such compounds is the corrosion of the apparatus (such as spin coaters) used for coating the substrate surface which results due to the chlorine atoms contained therein.
In this regard, a proposal has been made to use, as a dopant source of antimony, a coating liquid prepared by dissolving antimony sesquioxide (Sb.sub.2 O.sub.3) in an organic solvent. This method is, however, not quite as efficient as the diffusion coating method of antimony because antimony sesquioxide has a melting point and boiling point at 656.degree. C. and 1425.degree. C., respectively, while the diffusion process is usually performed at a temperature in the range from 1200.degree. to 1300.degree. C. Furthermore, the practical applicability of these coating liquids for diffusion of antimony is limited due to their poor storage stability. Therefore, it has been a long-sought goal of the semiconductor industry to develop an improved silica-based film-forming composition for diffusion of antimony free from the above described problems and disadvantages of the prior art compositions.